In recent years, an ultrasonic flowmeter has been used as a gas meter, for example. The ultrasonic flowmeter measures the time that it takes for an ultrasonic wave to go a predetermined distance within a tube in which a fluid is flowing, and calculates the flow velocity of the fluid, thereby obtaining its flow rate based on the flow velocity.
FIG. 35 illustrates a cross-sectional structure for a main portion of an ultrasonic flowmeter of that type. The ultrasonic flowmeter is arranged such that a fluid under measurement, of which the flow rate needs to be measured, can flow through a tube. A pair of ultrasonic transducers 101a and 101b is provided on the tube wall 102 so as to face each other. Each of the ultrasonic transducers 101a and 101b includes a piezoelectric vibrator, made of a piezoceramic, for example, as an electromechanical energy converter, and exhibits a resonance characteristic just like a piezoelectric buzzer or a piezoelectric oscillator.
In the state illustrated in FIG. 35, the ultrasonic transducer 101a is used as an ultrasonic transmitter and the ultrasonic transducer 101b is used as an ultrasonic receiver.
If an alternating current voltage, having a frequency in the vicinity of the resonant frequency of the ultrasonic transducer 101a, is applied to the piezoelectric body (i.e., piezoelectric vibrator) in the ultrasonic transducer 101a, then the ultrasonic transducer 101a functions as an ultrasonic transmitter to radiate an ultrasonic wave into the fluid. The ultrasonic wave radiated is propagated along a path L1 and reaches the ultrasonic transducer 101b. In this case, the ultrasonic transducer 101b functions as a receiver, which receives the ultrasonic wave and transforms it into a voltage.
Then, the ultrasonic transducer 101b functions as an ultrasonic transmitter and the ultrasonic transducer 101a functions as an ultrasonic receiver in turn. Specifically, when an alternating current voltage, having a frequency in the vicinity of the resonant frequency of the ultrasonic transducer 101b, is applied to the piezoelectric body in the ultrasonic transducer 101b, the ultrasonic transducer 101b radiates an ultrasonic wave into the fluid. The ultrasonic wave radiated is propagated along a path L2 and reaches the ultrasonic transducer 101a. The ultrasonic transducer 101a receives the ultrasonic wave propagated, and transforms it into a voltage.
In this manner, each of the ultrasonic transducers 101a and 101b alternately functions as a transmitter and as a receiver. Thus, these transducers 101a and 101b are normally called “ultrasonic transducers” or “ultrasonic transmitters-receivers” collectively.
In the ultrasonic flowmeter shown in FIG. 35, if the alternating current voltage is applied to one of the ultrasonic transducers continuously, then the ultrasonic transducer radiates an ultrasonic wave continuously, thereby making it difficult to measure the propagation time. For that reason, a burst voltage signal, which uses a pulse signal as a carrier, is normally used as a drive voltage.
Hereinafter, the measuring principle of this ultrasonic flowmeter will be described in further detail.
If an ultrasonic burst signal is radiated from the ultrasonic transducer 101a by applying the burst voltage signal to the ultrasonic transducer 101a for driving purposes, then the ultrasonic burst signal will be propagated along the path L1 and reach the ultrasonic transducer 101b in a time t. The distance of the path L1, as well as the path L2, is supposed to be L.
The ultrasonic transducer 101b can transform only the ultrasonic burst signal propagated into an electric burst signal at a high SNR. This electric burst signal is amplified electrically and then applied to the ultrasonic transducer 101a again, thereby making the ultrasonic transducer 101a radiate another ultrasonic burst signal. An apparatus performing such an operation is called a “sing-around type apparatus”. Also, a period of time it takes for an ultrasonic pulse, radiated from the ultrasonic transducer 101a, to reach the ultrasonic transducer 102b is called a “sing-around period”. The inverse number of the “sing-around period” is called a “sing-around frequency”.
In FIG. 35, the flow velocity of the fluid flowing through the tube is supposed to be V, the velocity of the ultrasonic wave in the fluid is supposed to be C, and the angle defined between the direction in which the fluid is flowing and the direction in which the ultrasonic pulse is propagated is supposed to be θ. If the ultrasonic transducers 101a and 101b are used as an ultrasonic transmitter and an ultrasonic receiver, respectively, then the following Equation (1) is satisfied:f1=1/t1=(C+V cos θ)/L  (1)where t1 is the sing-around period (i.e., the time it takes for the ultrasonic pulse, radiated from the ultrasonic transducer 101a, to reach the ultrasonic transducer 101b) and f1 is the sing-around frequency.
Conversely, if the ultrasonic transducers 101b and 101 are used as an ultrasonic transmitter and an ultrasonic receiver, respectively, then the following Equation (2) is satisfied:f2=1/t2=(C−V cos θ)/L  (2)where t2 is the sing-around period and f2 is the sing-around frequency in that situation.
The difference Δf between these two sing-around frequencies is given by:Δf=f1−f2=2V cos θ/L  (3)
According to Equation (3), the flow velocity V of the fluid can be obtained from the distance L of the ultrasonic wave propagation path and the frequency difference Δf. And the flow rate can be determined by the flow velocity V.
Such an ultrasonic flowmeter is required to exhibit high precision. To increase the precision, it is important to appropriately adjust the acoustic impedance of the acoustic matching layer that is provided on the ultrasonic wave transmitting/receiving surface of the piezoelectric body in the ultrasonic transducer. Particularly when the ultrasonic transducer radiates (i.e., transmits) an ultrasonic wave into a gas or receives an ultrasonic wave that has been propagated through a gas, the acoustic matching layer plays an important role.
Hereinafter, the role of the acoustic matching layer will be described with reference to FIG. 36. FIG. 36 shows a cross-sectional structure of a conventional ultrasonic transducer 103.
The ultrasonic transducer 103 shown in FIG. 36 includes a piezoelectric body 106, which is secured to an inside surface of a sensor case 105, and an acoustic matching layer 104, which is secured to an outside surface of the sensor case 105. Specifically, the acoustic matching layer 104 is bonded to the sensor case 105 with an adhesive such as an epoxy resin, and the piezoelectric body 106 is also bonded to the sensor case 105 by the same technique.
The ultrasonic vibrations of the piezoelectric body 106 are transmitted to the sensor case 106 by way of the adhesive layer and then to the acoustic matching layer 104 by way of the second adhesive layer. After that, the ultrasonic vibrations are radiated as acoustic waves into a gas that contacts with the acoustic matching layer 104 (which will be referred to herein as an “ultrasonic wave propagating medium”).
It is the role of the acoustic matching layer 104 to propagate the vibrations of the piezoelectric body to the gas efficiently. This point will be described in further detail below.
The acoustic impedance Z of a substance is defined by the following Equation (4):Z=ρ×C  (4)where C is the sonic velocity in the substance and σ is the density of the substance.
The acoustic impedance of a gas into which the ultrasonic wave is radiated is greatly different from that of the piezoelectric body. A piezoceramic such as lead zirconate titanate (PZT), which is a normal material for a piezoelectric body, has an acoustic impedance Z1 of about 30×106 kg/m2/s. On the other hand, the air has an acoustic impedance Z3 of about 400 kg/m2/s.
An acoustic wave is easily reflected from the boundary surface between two substances with mutually different acoustic impedances. Also, after having been transmitted through the boundary surface, the acoustic wave will have a decreased intensity. For that reason, a substance, of which the acoustic impedance Z2 is given by the following Equation (5), is inserted between the piezoelectric body and the gas:Z2=(Z1×Z3)1/2  (5)
If a substance having such an acoustic impedance Z2 is inserted, then the reflection from the boundary surface is decreased significantly and the transmittance of the acoustic wave increases.
If the acoustic impedance Z1 is 30×106 kg/m2/s and the acoustic impedance Z3 is 400 kg/m2/s, then the acoustic impedance Z2 that satisfies Equation (5) is about 11×104 kg/m2/s. The substance having the impedance of 11×104 kg/m2/s must satisfy Equation (4) (i.e., Z2=ρ×C). However, it is very difficult to find such a substance in solid materials. This is because such a substance is a solid but must have a sufficiently low density ρ and a low sonic velocity C.
A material obtained by solidifying a glass balloon or a plastic balloon with a resin material is currently used extensively as a material for the acoustic matching layer. Also, a technique of thermally compressing a hollow glass sphere and a technique of foaming a molten material are disclosed as exemplary methods for preparing such a material preferred for the acoustic matching layer in Japanese Patent No. 2559144, for example.
However, the acoustic impedance of each of these materials is greater than 50×104 kg/m2/s and cannot be regarded as satisfying Equation (5). To obtain a high-sensitivity ultrasonic transducer, the acoustic matching layer thereof needs to be made of a material with even smaller acoustic impedance.
To meet such a demand, the applicant of the present application invented an acoustic matching material satisfying Equation (5) fully and disclosed it in Japanese Patent Application No. 2001-056051. That material is made of a dry gel with durability and has a low density ρ and a low sonic velocity C.
An ultrasonic transducer, including an acoustic matching layer made of such a material with very low acoustic impedance (such as a dry gel), can transmit or receive an ultrasonic wave into/through a gas with high efficiency and sensitivity. As a result, an apparatus that can measure the flow rate of the gas with high precision is realized.
However, a material such as a dry gel with very low acoustic impedance normally has a low mechanical strength. In particularly, the dry gel is relatively strong with respect to a stress applied in the compressing direction but is extremely weak to a stress applied in the pulling or bending direction and is easily broken even under a weak impact.
Also, such a material has a very low sonic velocity. Accordingly, a preferred thickness of the acoustic matching layer (i.e., about a quarter of the transmission or reception wavelength) to achieve the maximum transmission or reception sensitivity becomes very small. For example, if an ultrasonic wave with a frequency of about 500 kHz is transmitted or received through a material with a sonic velocity of 60 m/s to 400 m/s, then the preferred thickness of the acoustic matching layer will be about 30 μm to about 200 μm. If the acoustic matching layer is so thin, it is extremely difficult to handle the acoustic matching layer as one member. In that case, it may be almost impossible to make an ultrasonic transducer by bonding such an acoustic matching layer to a sensor case or a piezoelectric body. Or even if it happened to be possible, it should be hard to use such an ultrasonic transducer actually in view of its production yield and cost.
Furthermore, since the acoustic matching layer has a low mechanical strength, the ultrasonic vibrations of the ultrasonic transducer in operation might cause peeling of the acoustic matching layer, thus possibly resulting in decreased reliability.
In order to overcome the problems described above, an object of the present invention is to provide an ultrasonic transducer, which includes an acoustic matching layer made of a material with a low mechanical strength and a low sonic velocity such as a dry gel but which can still be produced at a good yield and ensures high reliability, and also provide a method for fabricating such an ultrasonic transducer.
Another object of the present invention is to provide an ultrasonic flowmeter including such an ultrasonic transducer.